1. Field
Embodiments described herein relate to NAND-type flash memory with memory cells having a stacked gate structure.
2. Description of the Related Art
In current NAND-type flash memory, memory cells are individually provided with floating gate electrodes. Write and erase operations are performed by injecting and emitting electrons to and from these floating gate electrodes. Multiple threshold voltage states (data states) may be set by controlling the amount of electrons to be stored in the floating gate electrodes. Recently, such NAND-type flash memory has been developed and mass-produced that stores multiple bits in one memory cell, such as 2 bits (quaternary) or 3 bits (octal).
In this NAND-type flash memory, the threshold voltage of memory cell varies with the amount of electrons stored in the floating gate electrode. In a read operation, a certain voltage Vcgrv (which is intermediate between a plurality of threshold voltage distributions) is applied to a selected word line, while a read pass voltage Vread (which makes a memory cell conductive irrespective of data) is applied to unselected word lines. The read operation is performed by determining whether or not current flows through a NAND cell unit upon application of these voltages.
In order to allow sufficient current to flow through unselected memory cells, such a voltage is used as a read pass voltage Vread that is higher than the upper limit of the highest threshold voltage distribution by on the order of 2 V. As miniaturization trend continues, the potential of the floating gate electrode of a selected memory cell may rise beyond expectations due to the read pass voltage Vread applied to adjacent unselected word lines. This leads to a phenomenon in which the threshold voltage of a selected memory cell appears to decrease. As a result, a selected memory cell may erroneously become conductive, although the selected memory cell has a certain threshold voltage supposed to be non-conductive. This may cause incorrect data from a selected memory cell in read operation.